1. Field of the Invention
This invention relates in general to a direct conversion type transceiver in a communication system, and more particularly to an automatic gain control and offset correction technique used in a direct conversion type transceiver in a communication system.
2. Description of Related Art
Today""s wireless communications markets are being driven by a multitude of user benefits. Products such as cellular phones, cordless phones, pagers, and the like have freed corporate and individual users from their desks and homes and are driving the demand for additional equipment and systems to increase their utility. As a result digital radio personal communications devices will play an increasingly important role in the overall communications infrastructure in the next decade.
Mixed-signal integration and power management have taken on added importance now that analog and mixed analog-digital ICs have become the fastest-growing segment of the semiconductor industry. Integration strategies for multimedia consoles, cellular telephones and battery-powered portables are being developed, as well as applications for less integrated but highly specialized building blocks that serve multiple markets. These building blocks include data converters, demodulators, filters, amplifiers and voltage regulators.
One aspect of digital radio personal communications devices is the integration of the RF sections of transceivers. Compared to other types of integrated circuits, the level of integration in the RF sections of transceivers is still relatively low. Considerations of power dissipation, form factor, and cost dictate that the RF/IF portions of these devices evolve to higher levels of integration than is true at present. Nevertheless, there are some key barriers to realizing these higher levels of integration.
For example, in a time division duplex (TDD) frequency shift keying (FSK) transceiver, a direct conversion type of receiver architecture is often desired. The receiver accepts an incoming RF signal as RF positive (RF+) and RF negative (RFxe2x88x92), amplifies the signal with a low noise amplifier (LNA), and then directly converts the signal to a baseband frequency range. Channel select filters are typically used to select the desired channel that exits at frequencies, e.g. from 0 Hz (DC) to 800 KHz. Once the signal is filtered, the signal must be amplified and presented to a demodulator of the receiver for data recovery.
However, for information to be correctly received from the RF signal, the signal must be down-converted and amplified to a predetermined level, i.e. the signal paths must have a proper gain to present the signals to the demodulator. Such amplification results in a large amount of DC gain within the baseband. The large amount of DC gain acts upon small DC offsets within the various receiver components producing large DC offsets that destroy the desired signal.
In addition to the receiver having to deal with a large DC offset problem and simultaneous gain control, the constraint that the offset and gain loops must settle within a preamble period of the incoming signal, e.g. 32 bits or 320 uS, also exists.
Therefore, there is a need for incorporating an offset correction and automatic gain control system into a direct conversion type of transceiver in a communication system. Further, there is a need for an offset correction and automatic gain control system which is capable of controlling the offset and gain loops within a preamble period of the incoming signal.
It is with respect to these and other considerations that the present invention has been made.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an offset correction and automatic gain control system in a direct conversion type of transceiver in a communication system.
The present invention solves the above-described problems by canceling the DC offset and controlling the gain of the amplified signal within a preamble period of an incoming signal, whereby the incoming signal is first temporarily high pass filtered by means of a xe2x80x9cfeedback offset integrator,xe2x80x9d and second, all pass filtered by means of holding the offset integrator within the xe2x80x9cfeedback offset integrator,xe2x80x9d and third, peak detected over a certain time frame to determine its envelop. Note that the time frame of the envelop in conjunction with the signal format guarantees a positive peak and a negative peak to occur during the event of the xe2x80x9cenvelop detectionxe2x80x9d. The DC content of the incoming signal envelop is subtracted from the instantaneous incoming signal that contains a fixed DC offset, thereby resulting in the incoming signal without the fixed DC offset. The instantaneous incoming signal retains its necessary DC content while the undesired DC offset is cancelled.
The method in accordance with the principles of the present invention includes receiving the incoming RF signal; converting the RF signal to a baseband signal; amplifying the baseband signal; and canceling the DC offset and controlling the gain of the amplified baseband signal within a preamble period of the incoming signal, wherein the incoming baseband signal is peak detected over a certain time frame to determine its envelop. The DC content of the incoming baseband signal envelop is then subtracted from the instantaneous incoming baseband signal plus DC offset thereby resulting in the incoming baseband signal without DC offset. The instantaneous incoming baseband signal retains its necessary DC content while the undesired offset is cancelled.
Also, the present invention provides an offset correction and automatic gain control system. In one embodiment, the offset correction and automatic gain control system includes a feedback offset integrator which corrects the DC offset in a coarse fashion. The offset integrator is controlled by a timing mechanism during a preamble period of the incoming signal. A signal peak detector is used to determine the signal envelop. The envelop is used to further reduce the residual coarse fixed DC offset by means of a feed forward offset cancellation technique. In addition, the peak detector determined envelop is used to control the gain of various signal amplifiers and filters within the system.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the incoming signal is filtered to eliminate undesirable signals.
Another aspect of the present invention is that the direct conversion of the incoming RF signal includes converting the incoming RF signal to baseband in-phase (I) and quadrature (Q) signals. In one embodiment, the converted I and Q signals are differential input signals.
The present invention also provides for a direct conversion type transceiver system in a communication system. In one embodiment, the transceiver system includes: an antenna receiving an incoming RF signal; a low noise amplifier, coupled to the antenna, amplifying the signal; and an offset correction and automatic gain control system for correcting the DC offset and controlling the gain of the signal. The offset correction and automatic gain control system includes a feedback offset integrator which corrects the DC offset in a coarse fashion. The offset integrator is controlled by a timing mechanism during a preamble period of the incoming signal. A signal peak detector is used to determine the signal envelop. The envelop is used to further reduce the residual coarse fixed DC offset by means of a feed forward offset cancellation technique. In addition, the peak detector determined envelop is used to control the gain of various signal amplifiers and filters within the system.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.